The present invention relates to a magnetically soft, moldable composite material which contains powders that have magnetically soft properties and that have a nonmagnetic coating, and a method for its manufacture.
Magnetically soft materials are used for the manufacture of temperature-, corrosion-, and solvent-resistant magnetic components in the electronics sector, and in particular in electromechanics. These magnetically soft components generally have certain properties: high permeability (xcexcmax), high magnetic saturation (Bs), low coercivity field strength (Hc), and high specific electrical resistance (xcfx81spec). The combination of these magnetic properties with a high specific electrical resistance yields high switching dynamics; in other words, magnetic saturation and demagnetization of a component of this kind occur within a brief time period.
At present, soft iron plates, for example, are adhesively bonded into plate packets in order to serve as armatures of electric motors. Insulation of the plies is effective, however, in only one direction. European Patent No. 0 540 504 describes processing magnetically soft powders with a plastic binder and thereby manufacturing corresponding components using an injection-molding process. In order to guarantee the free-flowing capability necessary for injection molding, the powder components in injection-moldable composite materials are limited to a maximum of 65 vol %. On the other hand, densification of pourable powders under axial pressing, for example, is accomplished almost without material flow. The filling ratio of these composite materials is typically 90-98 vol %. The components shaped by axial pressing of powders are therefore characterized, in comparison to injection-molded ones, by considerably higher permeabilities and higher magnetic field strengths in the saturation range. Axial pressing of powders made of pure iron or nickel-iron with thermosetting resins, for example, epoxies or phenol resins, has the disadvantage, however, that the thermoplastic and thermosetting binders used hitherto are soluble or exhibit severe swelling at elevated temperature in organic solvents, for example, fuels for internal combustion engines. Under these conditions the corresponding composite components change dimensions, lose their strength, and fail completely. It was hitherto not possible to manufacture corresponding composite materials having good temperature and media resistance, for example, in organic solvents, in particular, in fuels for internal combustion engines. A further problem has hitherto been those utilization conditions for such components under which both thermoplastics and thermosetting resins no longer represent a suitable binder, since they would be completely decomposed.
The article by H. P. Baldus and M. Jansen in Angewandte Chemie, 1997, 109, pp. 338-394 describes modern high-performance ceramics which are formed from molecular precursors by pyrolysis, and in some cases also have magnetic properties. These ceramics are extremely stable with respect to temperature and solvents.
By coating magnetically soft powder grains with a nonmagnetic thermoplastic compound it is possible, advantageously, to increase the proportion of the magnetically soft powder in the composite material and, by the use of stable thermoplastic compounds, to achieve good temperature and solvent resistance for the shaped parts manufactured therefrom.
It is also particularly advantageous to coat a powder having magnetically soft properties with a silicon-containing compound which converts to a silicon-containing ceramic upon pyrolysis, thereby enhancing the coercivity field strength and decisively enhancing the temperature stability of a shaped part manufactured from that composite material.
Coating the magnetically soft powder with compounds of boron or of aluminum which convert upon pyrolysis into corresponding ceramics is a further preferred possibility for enhancing the solvent resistance and temperature resistance of the magnetically soft composite material and the shaped parts manufactured therefrom.
In an advantageous method for manufacturing a magnetically soft composite material, a thermoplastic compound is applied from a solution onto the powder grains. The powder grains are introduced into the polymer solution, and while the powder is in constant motion, the solvent is extracted at elevated temperature or under vacuum. The powder grains thereby receive a thin polymer coating in simple fashion, thus eliminating complex processes.
In the case of a coating with a material made from a precursor ceramic which contains either silicon, aluminum, or boron as principal components, after shaping of the material the temperature is advantageously selected so that the coating material converts into a ceramic, metallic, or even intermetallic end product, high magnetization and resistance to temperature and solvents being achieved.
In especially preferred fashion, the coating materials used are silicon compounds selected from the group consisting of binary hydrogen compounds of silicon, polydialkyl silanes, carbosilanes, polysilazanes, alkoxyalkyl silanes, alkyl polysiloxanes, alkyl silanols, and compounds of alkyl silanols with elements of the first main group. This ensures that a large class of molecular precursor compounds of silicon can be used, which upon pyrolysis make available various ceramics based on silicon-oxygen and also on silicon-nitrogen or silicon-nitrogen-oxygen, and can be optimized for the desired requirement profile. The corresponding ceramic, which also has an influence on the magnetic field strength and the switching time of the magnetically soft compounds, can be selected according to the applications of the component to be manufactured. It is also thereby possible to select the temperature range in accordance with the application.
In an equally preferred manner, boron compounds selected from the group consisting of borazol, pyridine or other xcfx80-donor boron adducts, for example borane-phosphane, borane-phosphinite, borane-sulfur, or boron-nitrogen adducts, boron silazanes, and polyborazanes, can be used to coat the magnetically soft powder, so that a variety of boron-containing ceramics can easily be made available after thermolysis.
It is also possible, in preferred fashion, to use as the aluminum precursor compound a polyalazane which can be used in very small quantities of 0.2 to 2 wt % in terms of the total portion weight. Aluminum-nitrogen ceramics are thereby produced as a coating for the magnetically soft powder, the proportion by weight of the magnetically soft powder being particularly high.